This invention relates generally to nuclear reactors and more particularly, to liquid metal nuclear reactors with passive decay heat removal systems.
Known liquid metal reactors typically include a radioactive primary system composed of a core, primary pump and intermediate heat exchanger (IHX) and a non-radioactive secondary sodium loop which transfers the heat gained in the IHX to a steam generator. The key primary system components (core, IHX, and primary pump) may be placed in a single large reactor vessel (pool arrangement) or they may be placed in separate vessels and connected together by piping (loop arrangement). In a Top Entry Loop (TEL) arrangement the primary piping is composed of very short inverted U-tube shaped pipes that are located above the component vessels in order to prevent a coolant loss in the event that the piping is breached. This arrangement also minimizes the length of primary sodium piping and the size (foot print) of the primary system itself.
In the loop concept the component vessels are connected together by the primary piping in a manner that circulates the coolant through the reactor core located in the reactor vessel to the IHX vessel and then to the pump vessel before it is returned to the core. This process is continuously repeated during operation in order to continue transferring core produced heat to the non-radioactive secondary sodium in the IHX. The reactor vessel and the satellite vessels are housed inside a concrete vault. The same flow path between the primary components is utilized in a pool type plant except that all of the components are located within the same vessel.
To protect the reactor structures from overheating following reactor shut down and loss of the normal shutdown heat removal system, a shutdown decay heat removal system is provided. This system may be composed of either: (1) redundant auxiliary liquid metal loops that transfer heat from small auxiliary IHX units to the atmosphere via auxiliary secondary sodium to air dump heat exchangers; or (2) by providing an air flow path that will allow natural circulating air to flow past the component vessel(s) where it removes the shutdown decay heat by convective heat transfer from the vessels while also cooling the primary vault. Residual heat removal systems are typically referred to as Decay Heat Removal Systems (DHRS). Passive DHRS include a plurality of pathways that permit outside air to enter the reactor silo and flow past the exterior of the primary vessel(s) and then exit the reactor silo carrying the decay heat to the atmosphere.
In the event of a reactor vessel leak, a loss of coolant accident is prevented by enclosing he sodium containing vessel(s) inside a separate close coupled guard vessel that also serves as the lower portion of the containment. A severe loss of coolant accident can not occur unless both the reactor and the guard or lower containment vessels fail at the same time.
In many locations around the world, nuclear power plants are located near the sea. This means that the components that are utilized in the DHRS which transfer decay heat from the reactor to the atmosphere are exposed to moist salt laden air. The moist salt air can increase the potential for corrosion within the DHRS. Especially if the temperature of the moist salt containing air is below the dew point, and salt water condenses on the DHRS components.
It would be desirable to provide a liquid metal nuclear reactor that includes a passive natural circulating DHRS that guards against increased corrosion from moist salt laden air and can also prevent a severe loss of coolant accident in the event that a double vessel breach occurs.
A liquid metal reactor is provided that includes in one embodiment a DHRS that prevents the incoming cooling air from dropping below the dew point before the air contacts critical reactor components. By maintaining the temperature of the incoming air above the dew point, an essential element in the corrosion process, the electrolyte, is not present in the DHRS and the potential for corrosion within the flowpaths of the DHRS is greatly reduced.
The reactor includes a concrete reactor vault, and at least one primary vessel located in the reactor vault and coupled to a reactor shield deck. Each primary vessel is substantially surrounded by a containment vessel in a spaced apart relationship. The reactor also includes a heat removal system which includes a guard vessel substantially surrounding each containment vessel in a spaced apart relationship, at least one inlet conduit in fluid communication with the ambient atmosphere outside the nuclear reactor, and at least one outlet conduit in fluid communication with the ambient atmosphere outside the nuclear reactor. A fluid flow heat transferring flowpath is formed by the inlet conduits, the space intermediate the guard vessel of each primary vessel and the containment vessel of each primary vessel, and the outlet conduits. The heat removal system also includes at least one heat exchanger in the flowpath to elevate the temperature of the air coolant so that the temperature remains above the dew point temperature as the air coolant flows through the flowpath.
The heat removal system also includes a second flowpath formed by at least one vault inlet conduit, the space intermediate the guard vessel and the concrete reactor silo, at least one vault outlet conduit, and at least one heat exchanger to maintain the temperature of the air coolant above the dew point temperature as the air coolant flows through the second flowpath.
The above describe liquid metal reactor eliminates the potential for corrosion within the ducts, pathways and structures which guide outside air past the reactor containment vessel(s) to transfer reactor core decay heat from the reactor to the ambient air outside the reactor by utilizing the removed heat to raise the temperature of the incoming air above the dew point. In addition, the guard vessel of the above described reactor will prevent a radiological release from occurring in the event that both the reactor and containment vessels fail. The reactor also maintains decay heat removal during such a double vessel leak by maintaining reactor vault cooling. Also, the reactor provides a lower cost method of supporting the reactor deck from which the primary vessel(s) are hung.